For you DIY reefers, here is how I made the screen mesh top:
2 - 48" screen aluminum framing material (HD, Lowes, Menards, etc)
4 - screen frame corners
1 - roll of spline material
1 - spline roller
1 - roll of screening material (see Jimsflies description above)
photos courtesy of beakerbob...
Cut the aluminum framing to the interior size of the top of the tank lip (minus 1/16"), insert the corner and snap together:
Lay the screening on the framework and, using the splining tool, press the spline into the groove of the frame, starting at one corner and working around:
The splining will keep the screen taunt. Trim the excess screen material off the frame:
And you have completed the screened tank top!